During the implementation of a photolithography step allowing an integrated circuit to be produced, a light source is used to illuminate a photolithography mask and thus project an image onto a resist layer placed on a substrate, often a semiconductor substrate. A pattern is then formed in the resist layer, and steps of implanting dopant atoms or etching steps may be implemented using the resist as a mask.
Generally, the pattern formed in the resist has a different geometry to that of the pattern present on the mask. This is because various optical effects can modify the geometry of the patterns formed in a resist layer. These effects, commonly called proximity effects, appear during the implementation of photolithography steps for forming small or closely spaced patterns. It has been proposed to modify the masks prior to the photolithography to take into account proximity effects, implementing what are called optical proximity correction (OPC) processes.
In an OPC process, a photolithography mask is designed allowing a desired target pattern to be obtained in a resist layer, despite proximity effects. This target pattern is a pattern that is conventionally designed using the circuit layout. Even if the target pattern is generally similar to this layout, it may be different to make producing the pattern in the resist easier, or also to make subsequent steps of etching or implantation of dopant atoms easier. Various rules are used to design the target pattern. By way of example, these rules may comprise geometric design rules—it is, for example, easier to produce wider lines.
Another exemplary rule is that of overetching. If the photolithography step is followed by an etching step that systematically etches too much material, this overetch may be taken into account by increasing the size of the resist patterns.
It is noted that the designed target may be checked to ensure that the resist pattern performs its function. It is, for example, recommended to check that there is no risk of undesired contact, for example electrical contact, being made, and that the desired, for example, electrical contacts are indeed provided for by the target pattern.
Once the target pattern has been designed by virtue of a set of rules, a simulation mask may be designed by implementing photolithography simulation steps to obtain a simulated resist pattern to compare with the target pattern. The simulated mask is then modified, and the simulation, comparison and modification steps are repeated. After a certain number of iterations, for example 10, a photolithography mask is obtained that provides a resist pattern similar to the target pattern.
By way of example, to implement an OPC process, the software package nmOPC from the company Mentor Graphics of Wilsonville, Oreg. may be used.
To improve the photolithography steps it has also been proposed to use light sources having a complex geometry, comprising, for example, several light spots. The light source to be used may then be defined as that which allows a resist pattern similar to the target pattern to be obtained. For this purpose, source mask optimization (SMO) processes, known to those skilled in the art, may be used. Various software packages allow a light source corresponding to a target pattern to be obtained. For example a target pattern may be made using the software package sold under the trade name Tachyon by the company Brion Technology of Santa Clara, Calif.
OPC or SMO processes, which allow the quality of implementation of a photolithography step to be improved, address drawbacks due to unexpected variations in the parameters of a photolithography step to be mitigated. Among these parameters are the luminous power, or dose, received by the resist, which may vary slightly, and also the focus. During the implementation of a photolithography step, in addition to the light source, the mask, and the resist layer, an optical system for forming an image in a focal plane located level with the resist, is used. If the wafer is shifted along the optical axis of this system, for example by a few tens of nanometers, the resist finds itself in a plane located out of the focal plane of the optical system. These focal variations may modify the luminous contrast received by the resist and therefore its polymerization, and consequently the geometry of the resist patterns obtained. Likewise, the light intensity delivered by the source may vary due to poorly controlled external parameters. The dose/focus process window is therefore generally spoken of by those skilled in the art.